The field of the present invention is fine mesh screening systems including the use of high frequency vibration.
Traditional vibratory screening structures typically include a base, a frame resiliently mounted on the base with a screen or screens extending across the frame. A low frequency vibratory drive in the speed range of 8 Hz to 30 Hz is mounted to the frame with eccentric weights. Specific vibratory motions are established in the frame by the low frequency vibratory drive, generating screen accelerations up to the 7 g range.
The foregoing devices have been used for screening fine materials and powders. Stainless steel woven mesh screens having interstices in the 30 to 150 micron range are used for such commercial processing. These delicate, woven meshes are typically thin and comparatively limp. The mesh is usually stretched tightly and attached to a screen frame. The vibration of such devices typically enhances gravity separation of particles presented to the screen. Where fine particles are to be screened, the vibration also has a deleterious effect in that the fine particles become suspended above a boundary layer over the vibrating screen.
Fine mesh screens can be reasonably fragile under many if not most applications. Backing screens and perforated plates have been used to help support such screens. Such supported screens may be bonded or unbonded. One type of area bonding is diffusion bonding. In some instances, a fine screen cloth, a coarse screen cloth and a perforated plate have been used. The separate layers are bonded by heat and pressure into a unitary structure. A frame is typically used about the periphery of the composite screen structure for support, mounting and screen tension.
In an effort to overcome the deficiency of low frequency vibration, high frequency vibration has been employed. Ultrasonic vibrators have been mounted to separator frames with a direct mechanical attachment to the screens at the centers thereof. Alternatively, ultrasonic drives have been bonded directly to the screen. Fine mesh, tensioned screens tend to suffer fatigue failure at the boundary layer with the mechanical coupling to the ultrasonic drive and to dissipate energy within a few inches of that coupling. Such devices also use low frequency vibratory drives, principally as a means for conveying material across the screen or screens.